2,3-Unsaturated carboxylic acids and esters can be prepared from the reaction of a formaldehyde (H2CO) source and a saturated carboxylic acid or ester containing one less carbon atom. Thus, acrylic and methacrylic acid derivatives can be prepared from the condensation of a formaldehyde source with acetic or propionic acid derivatives, respectively. The reaction produces one equivalent of water for each equivalent of carboxylic acid derivative reacted.
Although a number of catalysts have been proposed for this reaction, catalysts containing acidic vanadium and phosphorus oxides are among the most efficient, especially when a third component such as titanium or silicon is present in the catalyst. Water, however, tends to inhibit the condensation reaction with these catalysts. The use of formalin—which typically contains about 37 weight percent formaldehyde in water—as a starting material, therefore, is less efficient. Methanol can also be an inhibitor for the condensation reaction, and, since formalin can also contain methanol, the efficiency can be further lowered. When a carboxylic acid is the reactant, the presence of methanol in formalin can create a mixture of acids and methyl esters. And when an ester is the reactant, the water in formalin can create a mixture of acids and esters.
Industrial grade aqueous formaldehyde contains about 55 weight percent formaldehyde. It is relatively inexpensive and, therefore, is an economical source of this reactant. Thus, there is a need in the art for catalysts that are capable of condensing formaldehyde with alkanoic acids or esters in the vapor phase and that are tolerant of water in the feedstock. Ideally, such catalysts would also provide a high conversion of formaldehyde along with a high selectivity to the acrylic product.
Vanadium-titanium-phosphorus (V—Ti—P) mixed oxides are the best known catalysts for generating acrylic acid from the condensation of formaldehyde and acetic acid. But the preparation of these catalysts can be dangerous and is not amenable to scale-up. Typically, the titanium component is incorporated into these catalysts by first hydrolyzing liquid titanium chloride. This step, unfortunately, generates large quantities of hydrochloric acid fumes. Thus, there is also a need in the art for methods of preparing V—Ti—P mixed oxide catalysts that are safer and more amenable to industrial production.
The present invention addresses these needs as well as others that will be apparent from the following description and claims.